Are there cases whereby auto weights won't work "no matter what"?
There is non manifold mesh in your model. Go to edit mode, make sure you are in vertex select mode, deselect all, from the menu select->select all by trait->non manifold (shift-ctrl-alt-m). There's a lot of geometry in the head, lips, teeth, nose etc. If the model is not going to have complex facial animation or speech, delete all that wasted geometry inside the head. The other areas you will have to just attack piece by piece. Having selected the non manifold mesh you can ctrl-numpad+ a couple of times to select the surrounding verts, ctrl-i to select inverse and 'h' to hide most of the mesh. This will clear the field for you to work on isolated parts.